A mat is laid below a driver's seat or a passenger's seat of a vehicle. The mat is disposed on a carpet attached to a vehicle body panel. Patent Document 1 discloses a carpet fastener that includes a main body mounted to a carpet by a screw, a long mounting hole provided to a mat, and a rotating lid that intersects with the mounting hole to mount the mat to the main body. A rectangular column protruding upward is provided to the main body of the carpet fastener, and the rotating lid is rotatably supported by the distal end of the rectangular column The rotating lid is slightly smaller than the mounting hole of the mat, and the mat is mounted to the main body such that a user inserts the rotating lid into the mounting hole of the mat and rotates the rotating lid to make the rotating lid intersect with the mounting hole.